Mercury is just peeking into view in the dawn sky. The little planet is in the east in the waxing twilight, and looks like a bright star. It’ll stand highest in the sky on Friday. But because of the angle at which it rises, it’s hard to spot. In fact, from much of the United States, you probably can’t see it at all. The view is best from south of about Dallas.

That difficulty illustrates how tough it’s been for scientists to study Mercury. It’s never in view for long – no more than a couple of hours before sunrise or after sunset. And it’s so low in the sky that we always see it through a thick layer of air, so the view is murky – like trying to make out the shapes of clouds from the bottom of a swimming pool.

In the late 1800s and the early 1900s, astronomers did make a few crude maps of Mercury’s surface. But there was a lot they couldn’t figure out. That included the length of the planet’s day. At first, it appeared that Mercury completed one turn on its axis in 88 Earth days – the same length as its year.

In the early 1960s, though, astronomers bounced radio waves off the surface. That work showed that a day lasts 59 Earth days. So Mercury completes three turns on its axis for every two orbits around the Sun – three days for every two years.

Again, look quite low in the east not long before sunrise for elusive little Mercury – a planet that’s been hard to get to know.

Script by Damond Benningfield